Recent loss of sensitivity to summer temperature constrains tree growth synchrony among boreal Eurasian forests

Abstract

Abstract High-latitude terrestrial ecosystems are crucial to the global climate system and its regulation by vegetation. Since productivity of boreal forests is much limited by low summer temperatures, it is expected that trees subjected to warming are progressively decreasing their regional growth coherence in the last decades. In this study, we used a comprehensive network of indexed ring-width records to assess 20th-century spatiotemporal patterns of climatic sensitivity of forest growth around the Urals mountain range above 60\u2009°N (ca. 750,000\u2009km2). This area offers an excellent opportunity to test for warming effects as most north Eurasian conifers (including Larix, Picea and Pinus species) are found along a north-to-south temperature gradient across contrasting soil hydrothermal regimes (permafrost and permafrost-free). We observed positive associations between indexed ring-width and summer temperature over the past century, decreasing southwards. However, weaker (permafrost) or non-significant (permafrost-free) relationships were consistently found at the local and regional scales after 1960. A cointegration analysis indicated that tree-growth release from cold limitation significantly reduced the degree and spatial extent of synchronous growth at short- (annual) and long-term (decadal) scales, most likely by exposing forests to endogenous (local) factors (e.g., competition, soil properties, nutrient availability) and species-specific reactions. Whereas the loss of temperature sensitivity progressively reduced non-permafrost synchrony by 50% over the whole 20th century, permafrost forests decreased their synchrony only after the 1960s, by 20%. Radial growth was enhanced in permafrost sites, as suggested by increasing basal area increment. Our results unequivocally link a substantial decrease in temporal coherence of forest productivity in boreal ecosystems to a growth release from cold limitation that is concurrent with regional warming trends. This emerging pattern points to increasing dependence on local drivers of the carbon balance and the role as carbon sinks of forests in the northern Ural region.